In a cellular mobile communication system based on the same radio access technology (RAT), a number of base station units are remotely located, each of which constitutes a wireless communication area, thereby to provide a service area, wherein a mobile unit in one of the cells is allowed to have access to the base station unit through a radio channel. When the mobile unit moves from one cell to another during on-going conversation, the so-called handover HO is performed to permit the communication to be continued in a seamless manner.
There are two different types of handover, i.e., the intra-frequency handover (intra-freq HO) on one hand, and the inter-frequency handover (inter-freq HO) on the other.
In addition, in a cellular mobile communication system based on mutually different radio access technologies, there is the inter-RAT handover (inter-RAT HO) performed when a mobile unit moves across a cell-to-cell border based on mutually different radio access technologies.
The cell-to-cell handover under the same radio access technology, which may be called intra-RAT handover (intra-RAT HO), is in Contrast to the last-mentioned inter-RAT HO.
Referring to FIG. 12 illustrating the handover processing to be performed while the mobile unit is in motion, base station units BS1, BS2, BS3 and BS4 are separately located on a two-dimensional plane. These base station units BS1, BS2, BS3 and BS4 provide wireless communication links to mobile units through carrier waves at frequencies f1, f2, f3 and f4, respectively; and using radio access technologies RAT1, RAT1, RAT1 and RAT2, respectively.
Base station units BS1, BS2, BS3 and BS4 can be in communication with mobile units MS1, MS2, MS4 and MSG located in cell c1; with mobile units MS4 and MS5 located in cell c2, with mobile units MS2 and MS3 located in cell c3; and with mobile units MS6 and MS7 located in cell c4.
Mobile unit MS4 in motion between cells c1 and c2 performs the handover based on the intra-RAT-HO (and inter-freq-HO). Similarly, mobile unit MS2 in motion between cells c1 and c3 performs the handover based on the intra-RAT-HO (and intra-freq-HO); while mobile unit MS6 in motion between cells c1 and c4 performs the handover based on the inter-RAT-HO (and inter-freq-HO).
The well-known W-CDMA (wideband-code division multiple access) radio access technology provided by the 3GPP (3rd generation partnership project) has been in use for the third generation, cellular mobile communication systems as the standard radio access technology. For the W-CDMA system, the so-called compressed mode is provided to monitor or measure the performance of base station units operating at mutually different frequencies to provide the intra-RAT-HO (and inter-freq-HO) and/or inter-RAT-HO (and inter-freq-HO).
Under the above-mentioned situation, the base station unit sets a gap period, as show in FIG. 13(a), during which the data transmission through the dedicated channel DPCH is stopped. On the other hand, the mobile unit switches its frequency during the gap period, thereby to monitor the function of the base station unit operating at a different frequency.
In the 3GPP, the high speed downlink packet access HSDPA to realize for downlink a high speed packet transmission at a maximum transmission rate of 14.4 Mbps, which is an extension of W-CDMA wireless interface, has already been adopted as a technology standard (Non-Patent Document 2 referred to). In the adopted standard, high-speed downlink shared control channel HS-SCCH and high-speed physical downlink shared channel HS-PDSCH are additionally defined for downlink as independent channels separate from the above-mentioned dedicated channel to which the compressed mode is inherently applied. Similarly, high-speed dedicated physical control channel HS-DSPCCH is additionally defined for uplink.
In the HSDPA, adaptive modulation and coding scheme AMCS is adopted, which switches, depending on downlink channel quality indicator CQI indicative of the current state of the transmission paths for the respective mobile units, wireless transmission parameters such as data modulation scheme for the shared data channel, error correction scheme, coding rate for the error correction code, spreading factors for time/frequency domain, and the order of code-multiplexing of multicodes. In addition, hybrid automatic repeat request scheme HARQ is also adopted, under which a mobile unit sends the acknowledgement/negative acknowledgement ACK/NACK signals and the CQI signal hack to the base station unit through the dedicated control channel.
FIGS. 13(b) and 13(c) illustrate examples of packet signals transmitted from a base station unit to a mobile unit, with FIG. 13(b) showing a shared control channel for the base station unit-to-mobile unit transmission and FIG. 13(c) showing a shared data channel for the base station Unit-to-mobile unit transmission.
In the HSDPA, a mobile unit does not have, during the time period corresponding to the gap period, those packet data allotted thereto which are addressed to itself, because the exchange of data transmission with a base station unit cannot be performed if the base station unit operating at a different frequency is to be monitored or measured. The base station unit is therefore adapted to send, in advance of the provision of the gap period, instructions to the mobile unit to stop the allotment of data for a shared data channel through the shared control channel. In response to the instructions, the mobile unit provides the gap period, thereby to perform the monitoring and the measurement of the base station unit operating at a different frequency.
More specifically, in contrast to the situation of FIG. 13(a), wherein the base station unit provides the gap period by applying the data compression or the like to continuous data to be sent to a mobile unit, the gap period is provided in the case of FIGS. 13(b) and 13(c) by preventing the allotment of the packet control signal and the packet data for the mobile unit to the gap period.
It is to be noted here that the radio interface of W-CDMA- or HSDPA-based mobile communication system is generally referred to as universal terrestrial radio access UTRA.
Further study is now in progress for the evolved universal terrestrial radio access EUTRA and for the evolved universal terrestrial radio access network EUTRAN, both for the third generation radio access technology.
The orthogonal frequency division multiplexing access OFDMA has been proposed for providing the downlink for the SUTRA, while the AMOS technique has been applied to the OFDMA system as the EUTRA scheme (Non-Patent Documents 3 and 4 referred to). For the EUTRA scheme, the radio frame structure for the downlink transmission and a mapping method for the radio channel have been proposed (Non-Patent Document 4 referred to).
In regard to the intra-RAT-HO (and the intra-freq-HO) and/or the inter-RAT-HO (and the inter-freq HO) for the EUTRA/EUTRAN, an autonomous gap control method for autonomously providing the gap period when the instantaneous CQI value becomes lower than the mean CQI value has been proposed as a method for controlling the gap period to monitor or measure a different frequency-based base station unit (FIG. 1 of Non-Patent Document 5 referred to).
FIGS. 14(a) and 14(b) illustrate a method of controlling the gap period, which has been proposed in the past. In the prior-art method illustrated, the mobile unit receives the shared pilot channel, measures the instantaneous CQI values at a predetermined CQI measurement interval, and reports the measured CQI values to the base station unit. At the same time, the mobile unit averages the instantaneous CQI values at a predetermined interval (a system parameter) to provide mean CQI values, and then compare the mean CQI values with a CQI threshold value, which is also a system parameter. When the mean CQI value is lower than the CQI threshold value, the mobile unit sets itself in a measurement mode for monitoring or measuring the base station unit operating at a different frequency.
In the measurement mode, the mobile unit stops receiving the signals from the base station unit currently in communication, thereby to provide the gap period, when the measured instantaneous CQI value is lower than the mean CQI value. Upon receipt of the instantaneous CQI value from a certain mobile unit, the base station unit provides a mean CQI value for that mobile unit in a manner similar to the calculation at the mobile unit. The base station unit then compares the mean CQI value with a CQI threshold value, which is a system parameter. When the mean CQI value is higher than the CQI threshold value, the base station unit sets itself at an ordinary mode, while it sets itself at a measurement mode when the mean CQI value is lower than the CQI threshold value. In the measurement mode, the base station unit stops transmission of data packets to the mobile unit currently in communication therewith, to provide the gap period, when the measured instantaneous CQI value is lower than the mean CQI value.
As shown in FIG. 14(a), the mobile unit terminates the gap period to resume the measurement of instantaneous CQI values and the report to the base station unit, after the completion of the monitoring or measurement of the base station unit operating at a different frequency. A similar processing is repeatedly performed thereafter, as shown in FIG. 14(b), which illustrates the successive formation of a plurality of gap periods g1 to g6.
A next-generation mobile unit adapted to the EUTRA/EUTRAN is required to be operable in a plurality of mobile communication systems, which utilize mutually different radio access technologies. More specifically, such next-generation, mobile unit must be operable in mobile communication systems utilizing the UTRA, GSM (global system for mobile communication), or other radio access technologies, which are not specified even in the 3GPP standards. Such mobile communication systems may have a different frame length, different frame structure, and different means or processes for measuring the quality of signal reception at the mobile units. As a result, a mobile unit under control by a base station unit of the EUTRA/EUTRAN mobile communication system may not always be able to set, in the inter-RAO-HO (and the inter-freq-HO), the length of the gap period which is optimum to such radio access technology, due to the difference in minimum required gap length for the monitoring or measurement of a different radio access technology-based base station unit. When the length of the gap period is set at a value longer than a minimum required gap, that results in an unutilized portion in the gap period, adversely affecting the spectral efficiency as well as the time efficiency.
Non-Patent Document 1: Keiji Tachikawa “W-CDMA Mobile Communication System,” ISBN4-621-04894-5
Non-Patent Document 2: 3GPP TR technical Report 25.858 and HSDPA specification—related materials (http://www.3gpp.org/ftp/Specs/html-info/25-series.htm)
Non-Patent Document 3: 3GPP TR (Technical Report) 25.913, V2.1.0 (2005-05), Requirements for Evolved Universal Terrestrial Radio Access (UTRA) and Universal Terrestrial Radio Access Network (UTRAN). (http://www.3gpp.org/ftp/Spec/html-info/25913.htm)
Non-Patent Document 4: 3GPP TR (Technical Report) 25.814, V1.0.1 (2005-11), Physical Layer Aspects for Evolved UTRA. (http://www.3gpp.org/ftp/Specs/html-info/25814.htm)
Non-Patent Document 5: NTT DoCoMo, Inc. “Measurement for LTE Intra- and Inter-RAT Mobility,” 3GPP TSG RAN WG2 Meeting #50, Sophia Antipolis, France, 9-13 Jan. 2006